This invention relates to switches that respond to the change in capacity from a user touching a surface portion of the switch. The invention is especially adapted for a keyboard made up of a plurality of such capacity responsive switches.
Touch-responsive switches in the form of "capacitive glass" are common in the appliance field. This special glass has conductive elements fired on each side in a manner that forms a pair of series-connected capacitors having input and output terminals on the glass surface facing away from the user. A common plate of the two capacitors is formed on the outer glass surface facing the user. A high frequency oscillator applies pulses to the capacitors. A switching circuit connected with the capacitors interprets the presence of pulses as a no-input condition. When a user touches the capacitor plate on the outer surface of the glass, the high frequency pulses are shunted to ground through the user. The switch circuit interprets the absence of pulses as a positive input condition and responds by actuating an output device. Such a switch system is illustrated in U.S. Pat. No. 4,308,443, issued to Tucker et al.
Such "capacitive-glass" switch systems are not truly capacity responsive but require physical contact by the user with the electrically-conductive common plate of the series capacitors. This requires a metal film, such as tin oxide, be deposited on the outer surface of the glass. Such metal film is subject to abrasion by repeated cleaning and may even abrade to the point of erratic operation. A further problem with such system is that a failure of the high frequency oscillator causes a no-pulse condition, to which the switching circuit may detrimentally respond by energizing one or more outputs.
In my U.S. Pat. Nos. 4,731,548 and 4,758,735 I disclose touch control switch circuits which respond to a change in the capacity-to-ground of a plate member as a result of contact by the body of a user. Such circuits do not require a metal film on the surface portion contacted by the user so the problem of metal film abrasion is avoided. Further, the circuits disclosed in my patents are not subject to the catastrophic failure of erroneous output switching caused by the failure of an oscillator.